The present invention relates to automotive multi-slope frequency-modulated-continuous-wave (FMCW) radar systems, and more particularly to a method of associating data from a distributed object.
In FMCW radar systems, the frequency of the emitted radar energy is modulated with multiple frequency rate slopes, and the instantaneous difference in frequencies of the transmitted and reflected waveforms is determined to form a baseband signal including both time delay and Doppler frequency shift information. Thus, at least two frequency rate slopes are needed to determine the range (time delay) and range-rate (Doppler frequency shift) of a single target. However, multiple targets create multiple returns, and three or more frequency rate slopes are needed to identify the reflected waveforms corresponding to each target. In general, the baseband signal is processed using fast Fourier transformation (FFT) and frequency centroiding to identify target-related signal components and an association procedure is then used to associate the identified components with respective targets. Typically, the association procedure identifies the frequency components associated with a given target by determining if certain mathematical relationships linking properly associated components are satisfied. For example, given two potentially related frequency components, the frequency of a third related component can be mathematically predicted, and the presence or absence of a frequency component within a window or gate including on the predicted frequency can be determined. See, for example, the U.S. Pat. No. 5,191,337 to Brovko et al., issued on Mar. 2, 1993, and incorporated by reference herein.
However, when the emitted signal impinges on a distributed or extended target such as a truck or a guardrail, multiple scattering corners on the target produce multiple similar returns, frequently resulting in the presence of multiple frequency components in a single frequency gate. This ambiguity increases the likelihood of incorrectly associating the frequency components, which in turn, leads to errors in the range and range-rate calculations. The usual way of resolving the ambiguity is to select the frequency component that is closest to the predicted frequency; this criterion is referred to as minimum estimation error in Brovko et al. However, the minimum estimation error technique often results in selection of the wrong frequency component, particularly when baseband signal noise and/or errors in the frequency centroiding process shift one or more of the frequency components. Accordingly, what is needed is an improved association method that more reliably identifies frequency components associated with a given target, particularly in the presence of data ambiguity due to returns from a distributed or extended target containing multiple scattering corners.
The present invention is directed to an improved association method for a multiple-slope FMCW radar wherein candidate combinations of frequency components are identified as being correctly or incorrectly associated based on calculated range-rate. According to the invention, range and range-rate are calculated for various pairs of frequency components emanating from like-polarity slopes of the emitted radar signal, and pairs having similar ranges and range-rates are combined to form candidate associations. The range and range rate of the candidate associations are then computed using all four frequency components, and ambiguous candidate associations are resolved by selecting the candidate associations having similar range rates.